Early development will be studied in the mouse by the establishment of cell lineages in the pre-gastrulation embryo, and by the generation of insertional mutations affecting development in transgenic mice. In this work, mouse embryos will be infected with retroviruses at different stages of development. As a result, individual cells of the embryo will be labelled genetically, and the mosaicism thus generated will be used to follow cell lineages during early development. To facilitate tracking of the infected cells and their progeny, retroviral vectors introduced into mouse embryos will encode an easily identifiable and innocuous gene product, such as the bacterial beta galactosidase gene. Retroviruses will be introduced into preimplantation stage embryos, as well as in egg cylinder stage embryos, in order to establish a fate map of the mouse embryo prior to or at gastrulation. In particular, the origin of the germ line will be investigated. Mosaic animals will be bred with wild-type animals to generate transgenic strains containing a single proviral genome in the germ line. These strains will be analyzed to investigate the effect of parental imprinting on expression of the transgene, and to derive new retroviral-induced mutations. The proviruses will be used as tags to clone flanking genomic sequences and to identify the mutated genes. This approach should contribute to a better understanding of genes active during early development.